The present invention relates to countermeasures devices and more particularly to countermeasures devices wherein a sampled portion of the target pulse is used to construct a restored duplicate of the target pulse which is devoid of jamming.
Some commonly used types of synchronized pulse jamming signals can be quite effective against conventional pulse tracking radars by introducing tracking errors, by causing break-lock, or by causing tracking of false targets. For example, chaff can cause the radar's tracking gate to transfer from the target echo to the chaff echo. Pulse repeater jamming can be designed to interfere with range tracking through range gate pull-off; to interfere with angle tracking through modulation of the repeated pulse (countdown, false scan modulation, etc.); or to interfere with both range and angle tracking simultaneously.
Both rear-dispensed chaff (dispensed by an incoming target) and repeater jamming have the weakness (from the jammer viewpoint) that there is an inherent minimum time delay between the start of the target pulse and the start of the jamming pulse. In the case of rear-dispensed chaff, the time delay between the target pulse and the chaff echo is caused by an immediate decrease in chaff velocity after it is dispensed, and by the time required for the chaff to "bloom" to an appreciable size. The chaff echo therefore does not appear until somewhat after the start of the target pulse. In the case of repeater jamming, the time delay is caused by the transit time of the intercepted radar signal through the jammer and its receiving and transmitting RF transmission lines.
While this inherent time delay between the target pulse and the jamming pulse sometimes can be quite short, it represents a difference between the target signal and the jamming signal which can be exploited by a radar receiver to reject the jamming. One method for taking advantage of the minimum delay is to use a radar with extremely good range resolution (pulse width of 0.2 .mu.s or less). However, there are a large number of conventional tracking radars which are in use now, and which will continue to be in use for sometime, that have pulse widths greater than 0.2 .mu.s.
Previous efforts to take advantage of the minimum delay have used "leading-edge" circuits which reject the latter portion of the target pulse together with some part of the jamming pulse. Such techniques use RC, LC, or delay line video differentiation. When delay line video differentiation is used, a sufficient part of the target pulse must be preserved undistorted for processing by the tracking and display circuits. Therefore, the differentiation time constant cannot be much less than the duration of the target pulse. These efforts have been unsuccessful in providing radars with the capability of rejecting pulse deception jamming which has a short minimum delay.